This invention relates to corona charging arrangements and, more particularly, to improved AC corona charging arrangements.
The use of a corona discharge device to apply electric charges to a surface has been conventional in xerographic copiers since the inception of commercial xerography. Corona discharge devices include both small diameter wires and arrays of points which produce ions when a high voltage is applied. Originally, a DC voltage of several thousand volts was applied to a corona discharge device to ionize the adjacent air molecules, causing electric charges to be repelled from the device and attracted to an adjacent lower potential surface such as that of the photoreceptor to be charged. In the absence of control, however, such charging arrangements tend to deposit excessive and nonuniform charges on the adjacent surface.
In order to control the application of charges to the adjacent surface so as to provide a uniform charge distribution and avoid overcharging, a conductive screen has been interposed between the corona discharge device, sometimes referred to as a "coronode", and the surface to be charged. Such screened corona discharge devices are referred to as "scorotrons". Typical scorotron arrangements are described in the Walkup Pat. No. 2,777,957 and the Mayo Pat. No. 2,778,946. Early scorotrons, however, reduced the charging efficiency of the corona device to only about 3%. That is, only about three out of every one hundred ions generated at the corona wire reached the surface to be charged. They also exhibited poor control of charging uniformity and magnitude, sometimes allowing the surface to be charged to a voltage exceeding the screen potential by 100% or more. Improved scorotrons now in use usually control surface potentials to within about 3% of the reference voltage applied to the screen and operate at efficiencies of about 30% to 50% but they tend to be complex and correspondingly expensive. The Mott Pat. No. 3,076,092 discloses a DC biased AC corona/ charging arrangement which does not require a control screen.
Another corona discharge device contains a row, or two staggered rows, of pins to which a high voltage is applied to produce corona generating fields at the tips of the pins.
Because such corona discharge devices or "coronodes" ionize the oxygen and nitrogen molecules in the air, they usually generate ozone to an undesirable extent as well as nitrate compounds which tend to cause chemical corrosion. Usually, large charging devices are required to provide a high current capability while avoiding a tendency to produce arcing between the coronode wires and low voltage conductors of the charging device or the surface being charged at high charging rates.
Still another corona charging arrangement, called the "dicorotron", includes a glass-coated corona wire to which an AC voltage is applied and an adjacent DC electrode which drives charges of one polarity charge toward the photoreceptor to be charged while attracting the opposite polarity charges to itself. Dicorotrons, however, are fragile and expensive and, because of the much larger coated wire radius, require very high AC voltages (8-10 kV). They also generate high levels of ozone and nitrates and require substantial spacing of the corona wire from low voltage conducting elements and the surface to be charged in order to avoid arcing.
Negative corona emission from a conducting corona wire typically consists of concentrated points of electron emission and ionization which are randomly spaced along the corona wire. For reasons which are not yet completely understood, the spacing between these corona emission points or "hot spots" increases as relative humidity decreases which results in highly nonuniform charging of an adjacent surface. The spacing between the corona emission points also increases as the negative voltage applied to the corona wire is lowered toward the corona threshold voltage.
High quality xerographic imaging, particularly for the reproduction of images containing large areas of gray or of color in the medium range of equivalent neutral density, requires a high uniformity of charging along the length of the corona charging device with deviations in the charge per unit area applied to the adjacent surface of no more than plus or minus 3%. Scorotron charging devices of the type discussed above in which the surface potential of the photoreceptor is charged to about 2% of the final asymptote voltage within four time constants is highly desirable. Scorotrons, however, are inefficient, space consuming and are sensitive to dust collection. Moreover, the relatively low efficiency of scorotrons causes more ozone production than a more efficient charging system would generate.